8-Ring Small Pore Molecular Sieve as High Temperature SCR Catalyst

ABSTRACT

Described is a selective catalytic reduction catalyst comprising an iron-promoted 8-ring small pore molecular sieve. Systems and methods for using these iron-promoted 8-ring small molecular sieves as catalysts in a variety of processes such as abating pollutants in exhaust gases and conversion processes are also described.

PRIORITY

This application claims priority to pending patent application Ser.61/716,078 filed Oct. 19, 2012.

TECHNICAL FIELD

The present invention pertains to the field of selective catalyticreduction catalysts. More particularly, embodiments of the inventionrelate to selective catalytic reduction catalysts comprising an 8-ringsmall pore molecular sieve, and methods of using these catalysts in avariety of processes such as abating pollutants in exhaust gases.

BACKGROUND

Molecular sieves such as zeolites have been used extensively to catalyzea number of chemical reactions in refinery and petrochemical reactions,and catalysis, adsorption, separation, and chromatography. For example,with respect to zeolites, both synthetic and natural zeolites and theiruse in promoting certain reactions, including conversion of methanol toolefins (MTO reactions) and the selective catalytic reduction (SCR) ofnitrogen oxides with a reductant such as ammonia, urea or a hydrocarbonin the presence of oxygen, are well known in the art. Zeolites arecrystalline materials having rather uniform pore sizes which, dependingupon the type of zeolite and the type and amount of cations included inthe zeolite lattice, range from about 3 to 10 Angstroms in diameter.Zeolites having 8-ring pore openings and double-six ring secondarybuilding units, particularly those having cage-like structures haverecently found interest in use as SCR catalysts. A specific type ofzeolite having these properties is chabazite (CHA), which is a smallpore zeolite with 8 member-ring pore openings (˜3.8 Angstroms)accessible through its 3-dimensional porosity. A cage like structureresults from the connection of double six-ring building units by 4rings.

Catalysts employed in the SCR process ideally should be able to retaingood catalytic activity over the wide range of temperature conditions ofuse, for example, 200° C. to 600° C. or higher, under hydrothermalconditions. Hydrothermal conditions are often encountered in practice,such as during the regeneration of a soot filter, a component of theexhaust gas treatment system used for the removal of particles.

Metal-promoted zeolite catalysts including, among others, iron-promotedand copper-promoted zeolite catalysts, for the selective catalyticreduction of nitrogen oxides with ammonia are known. Iron-promotedzeolite beta (U.S. Pat. No. 4,961,917) has been an effective commercialcatalyst for the selective reduction of nitrogen oxides with ammonia.Unfortunately, it has been found that under harsh hydrothermalconditions, for example exhibited during the regeneration of a sootfilter with temperatures locally exceeding 700° C., the activity of manymetal-promoted zeolites begins to decline. This decline is oftenattributed to dealumination of the zeolite and the consequent loss ofmetal-containing active centers within the zeolite.

The synthesis of a zeolite varies according to structure type of thezeolite, but usually, zeolites are synthesized using a structuredirecting agent, sometimes referred to as a template or organictemplate) together with sources of silica and alumina. The structuredirecting agent can be in the form of an organic, i.e.tetraethylammonium hydroxide (TEAOH), or inorganic cation, i.e. Na⁺ orK⁺. During crystallization, the tetrahedral silica-alumina unitsorganize around the SDA to form the desired framework, and the SDA isoften embedded within the pore structure of the zeolite crystals.

Metal-promoted, particularly copper promoted aluminosilicate zeoliteshaving the CHA structure type and a silica to alumina molar ratiogreater than 1, particularly those having a silica to alumina ratiogreater than or equal to 5, 10, or 15 and less than about 1000, 500,250, 100 and 50 have recently solicited a high degree of interest ascatalysts for the SCR of oxides of nitrogen in lean burning enginesusing nitrogenous reductants. This is because of the wide temperaturewindow coupled with the excellent hydrothermal durability of thesematerials, as described in U.S. Pat. No. 7,601,662. Prior to thediscovery of metal promoted zeolites described in U.S. Pat. No.7,601,662, while the literature had indicated that a large number ofmetal-promoted zeolites had been proposed in the patent and scientificliterature for use as SCR catalysts, each of the proposed materialssuffered from one or both of the following defects: (1) poor conversionof oxides of nitrogen at low temperatures, for example 350° C. andlower, and (2) poor hydrothermal stability marked by a significantdecline in catalytic activity in the conversion of oxides of nitrogen bySCR. Thus, the invention described in U.S. Pat. No. 7,601,662 addresseda compelling, unsolved need to provide a material that would provideconversion of oxides of nitrogen at low temperatures and retention ofSCR catalytic activity after hydrothermal aging at temperatures inexcess of 650° C.

Even though the catalysts described in U.S. Pat. No. 7,601,662, exhibitexcellent properties, there is always a desire for improved performancein extended or different temperature windows. For example, for someapplications improved high temperature (e.g., temperatures exceeding450° C.) performance of Cu-SS-13 may be desired or required. To meetregulatory standards such as Euro 6 regulations and beyond, improvedperformance at high temperatures would be desirable.

SUMMARY

A first aspect of the present invention relates to a selective catalyticreduction catalyst comprising an 8-ring small pore molecular sievepromoted with greater than 5 wt. % iron so that the catalyst iseffective to catalyze the selective catalytic reduction of nitrogenoxides in the presence of a reductant at temperatures between 200° C.and 600° C.

In one or more embodiments, the iron-promoted 8-ring small poremolecular sieve is selected from the group consisting of iron-promotedzeolite having a structure type selected from AEI, AFT, AFX, CHA, EAB,ERI, KFI, LEV, SAS, SAT, and SAV. In one or more embodiments, theiron-promoted 8-ring small pore molecular sieve has the CHA crystalstructure.

In one or more embodiments, the iron-promoted 8-ring small poremolecular sieve having the CHA crystal structure is selected from analuminosilicate zeolite, a borosilicate, a gallosilicate, a SAPO, anALPO, a MeAPSO, and a MeAPO.

In one or more embodiments, the iron-promoted 8-ring small poremolecular sieve is selected from the group consisting of SSZ-13, SSZ-62,natural chabazite, zeolite K-G, Linde D, Linde R, LZ-218, LZ-235,LZ-236, ZK-14, SAPO-34, SAPO-44, SAPO-47, and ZYT-6.

In one or more embodiments, the iron-promoted 8-ring small poremolecular sieve is an aluminosilicate zeolite having the CHA crystalstructure and is selected from iron-promoted SSZ-13 and iron-promotedSSZ-62. In one or more embodiments, the zeolite has a silica to aluminaratio in the range of 5 and 100. In a specific embodiment, the silica toalumina ratio is in the range of 10 to 50.

In one or more embodiments the catalyst includes iron in the range of5.1 wt. % to 10 wt. %, calculated as Fe₂O₃.

Another aspect of the invention pertains to a catalytic articlecomprising the catalyst described above in a washcoat deposited on ahoneycomb substrate. The honeycomb substrate can comprise a wall flowfilter substrate or a flow through substrate.

Another aspect of the invention pertains to an exhaust gas treatmentsystem comprising the catalytic article described above disposeddownstream from a diesel engine and an injector that adds a reductant toan exhaust gas stream from the engine.

Another aspect of the invention pertains to a method for selectivelyreducing nitrogen oxides, the method comprising contacting a gaseousstream containing nitrogen oxides with a selective catalytic reductioncatalyst comprising an 8-ring small pore molecular sieve promoted withgreater than 5 wt. % iron to catalyze the selective catalytic reductionof the nitrogen oxides in the presence of a reductant at temperaturesbetween 200° and 600° C. In specific embodiments of the method, the8-ring small pore molecular sieve promoted with iron is selected fromthe group consisting of AEI, AFT, AFX, CHA, EAB, ERI, KFI, LEV, SAS,SAT, and SAV. In a more specific method embodiment, the 8-ring smallpore molecular sieve promoted with iron has the CHA crystal structure.In even more specific method embodiments, the 8-ring small poremolecular sieve promoted with iron and having the CHA crystal structureis selected from the group consisting of aluminosilicate zeolite, SAPO,ALPO and MeAPO.

In specific method embodiments, the 8-ring small pore molecular sievepromoted with iron and having the CHA crystal structure is selected fromthe group consisting of SSZ-13, SSZ-62, natural chabazite, zeolite K-G,Linde D, Linde R, LZ-218, LZ-235, LZ-236, ZK-14, SAPO-34, SAPO-44,SAPO-47, and ZYT-6. In more specific method embodiments, the 8-ringsmall pore molecular sieve promoted with iron and having the CHA crystalstructure is an aluminosilicate zeolite. In even more specific methodembodiments, the aluminosilicate zeolite is selected from SSZ-13 andSSZ-62.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows NO_(x) conversion versus temperature for two differentFe-SSZ-13 samples;

FIG. 2 shows the NO_(x) conversion versus temperature for SSZ-13 havingvarying Fe loadings;

FIG. 3 shows NO_(x) conversion versus temperature for SSZ-13 havingvarying Fe loadings; and

FIG. 4 shows N₂O generation versus temperature for SSZ-13 having varyingFe loadings.

DETAILED DESCRIPTION

Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways.

Embodiments of the invention are directed to catalysts includingmolecular sieves, methods for their preparation, catalytic articles,exhaust gas systems and methods for abating pollutants from exhaustgases using the catalysts.

With respect to the terms used in this disclosure, the followingdefinitions are provided. As used herein, molecular sieves refer tomaterials based on an extensive three-dimensional network of oxygen ionscontaining generally tetrahedral type sites and having a poredistribution. A zeolite is a specific example of a molecular sieve,further including silicon and aluminum. Reference to a“non-zeolite-support” or “non-zeolitic support” in a catalyst layerrefers to a material that is not a molecular sieve or zeolite and thatreceives precious metals, stabilizers, promoters, binders, and the likethrough association, dispersion, impregnation, or other suitablemethods. Examples of such non-zeolitic supports include, but are notlimited to, high surface area refractory metal oxides. High surface arearefractory metal oxide supports can comprise an activated compoundselected from the group consisting of alumina, zirconia, silica,titania, silica-alumina, zirconia-alumina, titania-alumina,lanthana-alumina, lanthana-zirconia-alumina, baria-alumina,baria-lanthana-alumina, baria-lanthana-neodymia-alumina,zirconia-silica, titania-silica, and zirconia-titania.

As used herein, the term “catalyst” refers to a material that promotes areaction. As used herein, the phrase “catalyst composition” refers to acombination of two or more catalysts, for example a combination of twodifferent materials that promote a reaction. The catalyst compositionmay be in the form of a washcoat. As used herein, the term “carrier”refers to a support that carries or supports a catalytic species such asa catalyzed honeycomb substrate.

As used herein, the term “substrate” refers to the monolithic materialonto which the carrier is placed, typically in the form of a washcoatcontaining a plurality of carriers having catalytic species thereon. Awashcoat is formed by preparing a slurry containing a specified solidscontent (e.g., 30-90% by weight) of carriers in a liquid vehicle, whichis then coated onto a substrate and dried to provide a washcoat layer.

As used herein, the term “washcoat” has its usual meaning in the art ofa thin, adherent coating of a catalytic or other material applied to asubstrate carrier material, such as a honeycomb-type carrier member,which is sufficiently porous to permit the passage of the gas streambeing treated.

In one or more embodiments, the substrate is a ceramic or metal having ahoneycomb structure. Any suitable substrate may be employed, such as amonolithic substrate of the type having fine, parallel gas flow passagesextending there through from an inlet or an outlet face of the substratesuch that passages are open to fluid flow there through. The passages,which are essentially straight paths from their fluid inlet to theirfluid outlet, are defined by walls on which the catalytic material iscoated as a washcoat so that the gases flowing through the passagescontact the catalytic material. The flow passages of the monolithicsubstrate are thin-walled channels, which can be of any suitablecross-sectional shape and size such as trapezoidal, rectangular, square,sinusoidal, hexagonal, oval, circular, etc. Such structures may containfrom about 60 to about 900 or more gas inlet openings (i.e. cells) persquare inch of cross section.

The ceramic substrate may be made of any suitable refractory material,e.g. cordierite, cordierite-α-alumina, silicon nitride, zircon mullite,spodumene, alumina-silica-magnesia, zircon silicate, sillimanite, amagnesium silicate, zircon, petalite, α-alumina, an aluminosilicate andthe like.

The substrates useful for the catalyst carriers of embodiments of thepresent invention may also be metallic in nature and be composed of oneor more metals or metal alloys. The metallic substrates may be employedin various shapes such as pellets, corrugated sheet or monolithic form.Specific examples of metallic substrates include the heat-resistant,base-metal alloys, especially those in which iron is a substantial ormajor component. Such alloys may contain one or more of nickel,chromium, and aluminum, and the total of these metals may advantageouslycomprise at least about 15 wt. % of the alloy, for instance, about 10 to25 wt. % chromium, about 1 to 8 wt. % of aluminum, and about 0 to 20 wt.% of nickel.

“Rich gaseous streams” including rich exhaust streams mean gas streamsthat have a λ<1.0.

“Rich periods” refer to periods of exhaust treatment where the exhaustgas composition is rich, i.e., has a λ<1.0.

“Rare earth metal components” refer to one or more oxides of thelanthanum series defined in the Periodic Table of Elements, includinglanthanum, cerium, praseodymium and neodymium. Rare earth metalcomponents can include at least one rare earth metal selected from Ce,Pr, Nd, Eu, Nb, Sm, Yb, and La.

“Alkaline earth component” refers to one or more chemical elementsdefined in the Periodic Table of Elements, including beryllium (Be),magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium(Ra).

“Alkali metal component” refers to one or more chemical elements definedin the Periodic Table of Elements, including lithium (Li), sodium (Na),potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).

One or more embodiments are directed to selective catalytic reductioncatalysts. The catalysts comprise an 8-ring small pore molecular sievepromoted with iron. The catalyst is effective to catalyst the selectivecatalytic reduction of nitrogen oxides in the presence of a reductant attemperatures in the range of 200° and 600° C. The molecular sieve havingthe 8-ring pore openings and double-six ring secondary building units,for example, those having the following structure types: AEI, AFT, AFX,CHA, EAB, ERI, KFI, LEV, SAS, SAT, and SAV. According to one or moreembodiments, it will be appreciated that by defining the molecularsieves by their structure type, it is intended to include the structuretype and any and all isotypic framework materials such as SAPO. ALPO andMeAPO materials having the same structure type.

In more specific embodiments, reference to an aluminosilicate zeolitestructure type limits the material to zeolites that do not includephosphorus or other metals substituted in the framework. Of course,aluminosilicate zeolites may be subsequently ion-exchanged with one ormore promoter metals such as iron, copper, cobalt, nickel, manganese,cerium, alkaline earth components or platinum group metals. However, tobe clear, as used herein, “aluminosilicate zeolite” excludesaluminophosphate materials such as SAPO, ALPO, and MeAPO materials, andthe broader term “zeolite” is intended to include aluminosilicates andaluminophosphates. In one or more embodiments, aluminosilicate zeoliteshave a silica to alumina mole ratio of 5 to 100, and in specificembodiments, 10 to 50, and in more specific embodiments 15-40.

In general, the SCR catalyst based on an 8-ring small pore molecularsieve promoted with iron should exhibit comparable NO_(x) conversionactivity with the catalysts of the state of the art. In general, thecatalyst should exhibit good NO_(x) conversion activity (NO_(x)conversion>50% over the range of 350° C. to 600° C. The NO_(x) activityis measured under steady state conditions at maximum NH₃-slip conditionsin a gas mixture of 500 ppm NO, 500 ppm NH₃, 10% O₂, 5% H₂O, balance N₂at a volume-based space velocity of 80,000 h⁻¹.

As used herein, the term “Na⁺-form of chabazite” refers to the calcinedform of this zeolite without any ion exchange. In this form, the zeolitegenerally contains a mixture of Na⁺ and H⁺ cations in the exchangesites. The fraction of sites occupied by Na⁺ cations varies depending onthe specific zeolite batch and recipe.

A molecular sieve can be zeolitic—zeolites—or non-zeolitic, and zeoliticand non-zeolitic molecular sieves can have the chabazite crystalstructure, which is also referred to as the CHA structure by theInternational Zeolite Association. Zeolitic chabazite include anaturally occurring tectosilicate mineral of a zeolite group withapproximate formula: (Ca,Na₂,K₂,Mg)Al₂Si₄O₁₂.6H₂O (e.g., hydratedcalcium aluminum silicate). Three synthetic forms of zeolitic chabaziteare described in “Zeolite Molecular Sieves,” by D. W. Breck, publishedin 1973 by John Wiley & Sons, which is hereby incorporated by reference.The three synthetic forms reported by Breck are Zeolite K-G, describedin J. Chem. Soc., p. 2822 (1956), Barrer et al; Zeolite D, described inBritish Patent No. 868,846 (1961); and Zeolite R, described in U.S. Pat.No. 3,030,181, which are hereby incorporated by reference. Synthesis ofanother synthetic form of zeolitic chabazite, SSZ-13, is described inU.S. Pat. No. 4,544,538, which is hereby incorporated by reference.Synthesis of a synthetic form of a non-zeolitic molecular sieve havingthe chabazite crystal structure, silicoaluminophosphate 34 (SAPO-34), isdescribed in U.S. Pat. No. 4,440,871 and No. 7,264,789, which are herebyincorporated by reference. A method of making yet another syntheticnon-zeolitic molecular sieve having chabazite structure, SAPO-44, isdescribed in U.S. Pat. No. 6,162,415, which is hereby incorporated byreference.

In one or more embodiments, the 8-ring small pore molecular promotedwith iron is selected from the group consisting of AEI, AFT, AFX, CHA,EAB, ERI, KFI, LEV, SAS, SAT, and SAV. In a more specific embodiment,the 8-ring small pore molecular sieve promoted with iron can include allaluminosilicate, borosilicate, gallosilicate, MeAPSO, and MeAPOcompositions. These include, but are not limited to SSZ-13, SSZ-62,natural chabazite, zeolite K-G, Linde D, Linde R, LZ-218, LZ-235.LZ-236, ZK-14, SAPO-34, SAPO-44, SAPO-47, and ZYT-6. However, inspecific embodiments, the 8-ring small pore molecular sieve will havethe aluminosilicate composition, such as SSZ-13 and SSZ-62, which wouldexclude borosilicate, gallosilicate, MeAPSO, SAPO and MeAPOcompositions.

In one or more embodiments, iron-promoted 8-ring small pore molecularsieve has the CHA crystal structure and is selected from the groupconsisting of aluminosilicate zeolite having the CHA crystal structure,SAPO, ALPO, and MeAPO. In particular, the iron-promoted 8-ring smallpore molecular sieve having the CHA crystal structure is aniron-promoted aluminosilicate zeolite having the CHA crystal structure.In a specific embodiment, the iron-promoted 8-ring small pore molecularsieve having the CHA crystal structure will have an aluminosilicatecomposition, such as SSZ-13 and SSZ-62. In a very specific embodiment,the iron-promoted 8-ring small pore molecular sieve having the CHAcrystal structure is SSZ-13.

Wt % Iron:

The iron-promoted 8-ring small pore molecular sieve comprises greaterthan 5% by weight iron. The Fe content of the 8-ring small poremolecular sieve promoted with iron, calculated as Fe₂O₃, in specificembodiments is at least about 5.1 wt. %, and in even more specificembodiments at least about 5.5 or 6 wt. % reported on a volatile-freebasis. In even more specific embodiments, the Fe content of the 8-ringsmall pore molecular sieve promoted with iron, calculated as Fe₂O₃, isin the range of from about 5.1 wt. %, to about 15 wt. %, or to about 10wt. %, more specifically to about 9 wt. %, and even more specifically toabout 8 wt. %, in each case based on the total weight of the calcinedmolecular sieve reported on a volatile free basis. Therefore, inspecific embodiments, ranges of the 8-ring small pore molecular sievepromoted with iron, calculated as Fe₂O₃, are from about 5.1 to 15, 5.1to 10, 5.1 to 9, 5.1 to 8, 5.1 to 7, 5.1 to 6, 5.5 to 15, 5.5 to 10, 5.5to 9, 5.5 to 8, 5.5, 6 to 15, 6 to 10, 6 to 9, and 6 to 8 wt. %. All wt.% values are reported on a volatile free basis.

In one or more embodiments, the iron is exchanged into the 8-ring smallpore molecular sieve.

SCR Activity:

In specific embodiments, the 8-ring small pore molecular sieve promotedwith iron exhibits an aged NO_(x) conversion at 350° C. of at least 50%measured at a gas hourly space velocity of 80000 h⁻¹. In specificembodiments the 8-ring small pore molecular sieve promoted with ironexhibits an aged NO_(x) conversion at 450° C. of at least 70% measuredat a gas hourly space velocity of 80000 h⁻¹. More specifically the agedNO_(x) conversion at 350° C. is at least 55% and at 450° C. at least75%, even more specifically the aged NO_(x) conversion at 350° C. is atleast 60% and at 550° C. at least 80%, measured at a gas hourlyvolume-based space velocity of 80000 h⁻¹ under steady state conditionsat maximum NH₃-slip conditions in a gas mixture of 500 ppm NO, 500 ppmNH₃, 10% O₂, 5% H₂O, balance N₂. The cores were hydrothermally aged in atube furnace in a gas flow containing 10% H₂O, 10% O₂, balance N₂ at aspace velocity of 4,000 h⁻¹ for 5 h at 750° C.

The SCR activity measurement has been demonstrated in the literature,for example WO 2008/106519.

Sodium Content:

In specific embodiments, the 8-ring small pore molecular sieve promotedwith iron has a sodium content (reported as Na₂O on a volatile freebasis) of below 2 wt. %, based on the total weight of the calcinedmolecular sieve. In more specific embodiments, sodium content is below 1wt. %, even more specifically below 2000 ppm, even more specificallybelow 1000 ppm, even more specifically below 500 ppm and mostspecifically below 100 ppm.

Na:Al:

In specific embodiments, the 8-ring small pore molecular sieve promotedwith iron has an atomic sodium to aluminum ratio of less than 0.7. Inmore specific embodiments, the atomic sodium to aluminum ratio is lessthan 0.35, even more specifically less than 0.007, even morespecifically less than 0.03 and even more specifically less than 0.02.

Conventional Zeolite Synthesis of CHA-Type Molecular Sieves

In what may be referred to as a conventional synthesis of an 8-ringsmall pore molecular sieve having the CHA structure, a source of silica,a source of alumina, and a structure directing agent are mixed underalkaline aqueous conditions. Typical silica sources include varioustypes of fumed silica, precipitated silica, and colloidal silica, aswell as silicon alkoxides. Typical alumina sources include boehmites,pseudo-boehmites, aluminum hydroxides, aluminum salts such as aluminumsulfate or sodium aluminate, and aluminum alkoxides. Sodium hydroxide istypically added to the reaction mixture. A typical structure directingagent for this synthesis is adamantyltrimethyl ammonium hydroxide,although other amines and/or quaternary ammonium salts may besubstituted or added to the latter directing agent. The reaction mixtureis heated in a pressure vessel with stirring to yield the crystallineSSZ-13 product. Typical reaction temperatures are in the range of 100and 200° C., and in specific embodiments between 135 and 170° C. Typicalreaction times are between 1 hr and 30 days, and in specificembodiments, between 10 hours and 3 days.

At the conclusion of the reaction, optionally the pH is adjusted tobetween 6 and 10, and in specific embodiments, between 7 and 7.5, andthe product is filtered and washed with water. Any acid can be used forpH adjustment, and in specific embodiments nitric acid is used.Alternatively, the product may be centrifuged. Organic additives may beused to help with the handling and isolation of the solid product.Spray-drying is an optional step in the processing of the product. Thesolid product is thermally treated in air or nitrogen. Alternatively,each gas treatment can be applied in various sequences, or mixtures ofgases can be applied. Typical calcination temperatures are in the 400°C. to 850° C. range.

Optionally NH₄-Exchange to Form NH₄-Chabazite:

Optionally, the obtained alkali metal zeolite is NH₄-exchanged to formNH₄-Chabazite. The NH₄-ion exchange can be carried out according tovarious techniques known in the art, for example Bleken, F.; Bjorgen,M.; Palumbo, L.; Bordiga, S.; Svelle, S.; Lillerud, K.-P.; and Olsbye,U. Topics in Catalysis 52, (2009), 218-228.

Synthesis of CHA-Type Zeolites According to Embodiments of the Invention

According to one or more embodiments, methods for the synthesis ofselective catalytic reduction catalysts comprising an 8-ring small poremolecular sieve promoted with iron are provided. Particularly, thecatalyst comprises iron-promoted SSZ-13. The synthesis of iron-promotedCHA-type zeolites, particularly CHA-type aluminosilicate zeolites suchas SSZ-13 and SSZ-62 are provided.

Generally, preparation of the iron-promoted 8-ring small pore molecularsieve starts by calcination of ammonium form zeolite, followed byconventional liquid ion exchange using Fe precursor salt at 60° C. for 2hours at pH 4.5, which may require a buffer. The resultant product isfiltered, washed, air dried or spray dried. In other embodiments, theiron is exchanged directly into a Na form of the molecular sieve usingFe precursor salt at 60° C. for 1-2 hours in the presence of a buffer.The dried product is used to prepare a slurry and is coated onto aceramic flow-through honeycomb. Alternatively, an in-situ method can beused to prepare the iron-promoted 8-ring small pore molecular sieve. Forexample, an appropriate concentration of Fe salt solution is addeddrop-wise to a slurry of Hydrogen or ammonium form SSZ-13. BET:

In specific embodiments, the 8-ring small pore molecular sieve promotedwith iron exhibits a BET surface area, determined according to DIN66131, of at least about 400 m²/g, more specifically of at least about550 m²/g, even more specifically of at about 650 m²/g. In specificembodiments, the 8-ring small pore molecular sieve promoted with ironexhibits a BET surface area in the range from about 400 to about 750m²/g, more specifically from about 500 to about 750 m²/g.

In specific embodiments, the crystallites of the calcined the 8-ringsmall pore molecular sieve promoted with iron have a mean length in therange of from 10 nanometers to 100 micrometers, specifically in therange of from 50 nanometers to 5 micrometers, as determined via SEM. Inmore specific embodiments, the molecular sieve crystallites have a meanlength greater than 0.5 microns or 1 micron, and less than 5 microns.

Shape:

The 8-ring small pore molecular sieve promoted with iron according toembodiments of the invention may be provided in the form of a powder ora sprayed material obtained from above-described separation techniques,e.g. decantation, filtration, centrifugation, or spraying. In general,the powder or sprayed material can be shaped without any othercompounds, e.g. by suitable compacting, to obtain moldings of a desiredgeometry, e.g. tablets, cylinders, spheres, or the like.

By way of example, the powder or sprayed material is admixed with orcoated by suitable modifiers well known in the art. By way of example,modifiers such as silica, alumina, zeolites or refractory binders (forexample a zirconium precursor) may be used. The powder or the sprayedmaterial, optionally after admixing or coating by suitable modifiers,may be formed into a slurry, for example with water, which is depositedupon a suitable refractory carrier (for example WO 2008/106519).

The 8-ring small pore molecular sieve promoted with iron according toembodiments of the invention may also be provided in the form ofextrudates, pellets, tablets or particles of any other suitable shape,for use as a packed bed of particulate catalyst, or as shaped piecessuch as plates, saddles, tubes, or the like.

In specific embodiments, the 8-ring small pore molecular sieves aresubstantially comprised of alumina and silica and have a silica toalumina ratio in the range of about 1 to 1000, and in specificembodiments from 1 to 500, and in more specific embodiments from 5 to300, 10 to 200, 10 to 100, 10 to 90, 10 to 80, 10 to 70, 10 to 60, 10 to50, 10 to 40, 10 to 35 and 10 to 30 are within the scope of theinvention. In specific embodiments, the 8-ring small pore molecularsieve is iron-promoted SSZ-13 and/or iron-promoted SSZ-62.

In general, the 8-ring small pore molecular sieve promoted with irondescribed above can be used as a molecular sieve, adsorbent, catalyst,catalyst support or binder thereof. In especially specific embodiments,the material is used as a catalyst.

Moreover, embodiments of the invention relates to a method of catalyzinga chemical reaction wherein the 8-ring small pore molecular sievepromoted with iron according to embodiments of the invention is employedas catalytically active material.

Among others, said catalyst may be employed as a catalyst for theselective reduction (SCR) of nitrogen oxides (NO_(x)); for the oxidationof NH₃, in particular for the oxidation of NH₃ slip in diesel systems;for the decomposition of N₂O; for soot oxidation; for emission controlin Advanced Emission Systems such as Homogeneous Charge CompressionIgnition (HCCI) engines; as additive in fluid catalytic cracking (FCC)processes; as catalyst in organic conversion reactions; or as catalystin “stationary source” processes. For applications in oxidationreactions, in specific embodiments an additional precious metalcomponent is added to the copper chabazite (e.g. Pd, Pt).

Therefore, embodiments of the invention also relate to a method forselectively reducing nitrogen oxides (NO_(x)) by contacting a streamcontaining NO_(x) with a catalyst containing the 8-ring small poremolecular sieve promoted with iron according to embodiments of theinvention under suitable reducing conditions; to a method of oxidizingNH₃, in particular of oxidizing NH₃ slip in diesel systems, bycontacting a stream containing NH₃ with a selective catalytic reductioncatalyst comprising an 8-ring small pore molecular sieve promoted withiron according to embodiments of the invention under suitable oxidizingconditions; to a method of decomposing of N₂O by contacting a streamcontaining N₂O with a selective catalytic reduction catalyst comprisingan 8-ring small pore molecular sieve promoted with iron according toembodiments of the invention under suitable decomposition conditions; toa method of controlling emissions in Advanced Emission Systems such asHomogeneous Charge Compression Ignition (HCCI) engines by contacting anemission stream with a selective catalytic reduction catalyst comprisingan 8-ring small pore molecular sieve promoted with copper and the 8-ringsmall pore molecular sieve promoted with iron according to embodimentsof the invention under suitable conditions; to a fluid catalyticcracking FCC process wherein the selective catalytic reduction catalystcomprising an 8-ring small pore molecular sieve promoted with iron isemployed as additive; to a method of converting an organic compound bycontacting said compound with a selective catalytic reduction catalystcomprising an 8-ring small pore molecular sieve promoted with ironaccording to embodiments of the invention under suitable conversionconditions; to a “stationary source” process wherein a catalyst isemployed containing the 8-ring small pore molecular sieve promoted withiron according to embodiments of the invention.

In particular, the selective reduction of nitrogen oxides wherein theselective catalytic reduction catalyst comprising an 8-ring small poremolecular sieve promoted with iron according to embodiments of theinvention is employed as catalytically active material is carried out inthe presence of ammonia or urea. While ammonia is the reducing agent ofchoice for stationary power plants, urea is the reducing agent of choicefor mobile SCR systems. Typically, the SCR system is integrated in theexhaust gas treatment system of a vehicle and, also typically, containsthe following main components: selective catalytic reduction catalystcomprising the 8-ring small pore molecular sieve promoted with ironaccording to embodiments of the invention; a urea storage tank; a ureapump; a urea dosing system; a urea injector/nozzle; and a respectivecontrol unit.

Method of Reducing NO_(x):

Therefore, embodiments of the invention also relate to a method forselectively reducing nitrogen oxides (NO_(x)), wherein a gaseous streamcontaining nitrogen oxides (NO_(x)), for example exhaust gas formed inan industrial process or operation, and in specific embodiments alsocontaining ammonia and/or urea, is contacted with the selectivecatalytic reduction catalyst comprising the 8-ring small pore molecularsieve promoted with iron according to embodiments of the invention.

The term nitrogen oxides, NO_(x), as used in the context of embodimentsof the invention designates the oxides of nitrogen, especiallydinitrogen oxide (N₂O), nitrogen monoxide (NO), dinitrogen trioxide(N₂O₃), nitrogen dioxide (NO₂), dinitrogen tetroxide (N₂O₄), dinitrogenpentoxide (N₂O₅), nitrogen peroxide (NO₃).

The nitrogen oxides which are reduced using a selective catalyticreduction catalyst comprising the 8-ring small pore molecular sievepromoted with iron according to embodiments of the invention or an8-ring small pore molecular sieve promoted with iron obtainable orobtained according to embodiments of the invention may be obtained byany process, e.g. as a waste gas stream. Among others, waste gas streamsas obtained in processes for producing adipic acid, nitric acid,hydroxylamine derivatives, caprolactame, glyoxal, methyl-glyoxal,glyoxylic acid or in processes for burning nitrogenous materials may bementioned.

In especially specific embodiments, a selective catalytic reductioncatalyst comprising the 8-ring small pore molecular sieve promoted withiron according to embodiments of the invention or the 8-ring small poremolecular sieve promoted with with iron obtainable or obtained accordingto embodiments of the invention is used for removal of nitrogen oxides(NO_(x)) from exhaust gases of internal combustion engines, inparticular diesel engines, which operate at combustion conditions withair in excess of that required for stoichiometric combustion, i.e.,lean.

Therefore, embodiments of the invention also relate to a method forremoving nitrogen oxides (NO_(x)) from exhaust gases of internalcombustion engines, in particular diesel engines, which operate atcombustion conditions with air in excess of that required forstoichiometric combustion, i.e., at lean conditions, wherein a selectivecatalytic reduction catalyst comprising the 8-ring small pore molecularsieve promoted with iron according to embodiments of the invention or an8-ring small pore molecular sieve promoted with iron obtainable orobtained according to embodiments of the invention is employed ascatalytically active material.

Exhaust Gas Treatment System:

Embodiments of the invention relate to an exhaust gas treatment systemcomprising an exhaust gas stream optionally containing a reductant suchas ammonia, urea and/or hydrocarbon, and in specific embodiments,ammonia and/or urea, and a selective catalytic reduction catalyticarticle containing the 8-ring small pore molecular sieve promoted withiron, disposed on a substrate, and a second exhaust gas treatmentcomponent, for example, a soot filter and a diesel oxidation catalyst.

The soot filter, catalyzed or non-catalyzed, may be upstream ordownstream of said catalytic article. The diesel oxidation catalyst inspecific embodiments is located upstream of said catalytic article. Inspecific embodiments, said diesel oxidation catalyst and said catalyzedsoot filter are upstream from said catalytic article.

In specific embodiments, the exhaust is conveyed from the diesel engineto a position downstream in the exhaust system, and in more specificembodiments, containing NO_(x), where a reductant is added and theexhaust stream with the added reductant is conveyed to said catalyticarticle.

For example, a catalyzed soot filter, a diesel oxidation catalyst and areductant are described in WO 2008/106519 which is incorporated byreference. In specific embodiments, the soot filter comprises awall-flow filter substrate, where the channels are alternately blocked,allowing a gaseous stream entering the channels from one direction(inlet direction), to flow through the channel walls and exit from thechannels from the other direction (outlet direction).

An ammonia oxidation catalyst may be provided downstream of thecatalytic article to remove any slipped ammonia from the system. Inspecific embodiments, the AMOX catalyst may comprise a platinum groupmetal such as platinum, palladium, rhodium or combinations thereof. Inmore specific embodiment, the AMOX catalyst can include a washcoatcontaining the 8-ring small pore molecular sieve promoted with iron.

Such AMOX catalysts are useful in exhaust gas treatment systemsincluding an SCR catalyst. As discussed in commonly assigned U.S. Pat.No. 5,516,497, the entire content of which is incorporated herein byreference, a gaseous stream containing oxygen, nitrogen oxides andammonia can be sequentially passed through first and second catalysts,the first catalyst favoring reduction of nitrogen oxides and the secondcatalyst favoring the oxidation or other decomposition of excessammonia. As described in U.S. Pat. No. 5,516,497, the first catalystscan be a SCR catalyst comprising a zeolite and the second catalyst canbe an AMOX catalyst comprising a zeolite.

AMOX and/or SCR catalyst composition can be coated on the flow throughor wall-flow filter. If a wall flow substrate is utilized, the resultingsystem will be able to remove particulate matter along with gaseouspollutants. The wall-flow filter substrate can be made from materialscommonly known in the art, such as cordierite, aluminum titanate orsilicon carbide. It will be understood that the loading of the catalyticcomposition on a wall flow substrate will depend on substrate propertiessuch as porosity and wall thickness, and typically will be lower thanloading on a flow through substrate.

Ion Exchange of Metal:

In order to promote the SCR of oxides of nitrogen, a suitable metal isexchanged into the zeolite material. Suitable metals include, but arenot limited to copper, iron, cobalt, nickel, manganese, cerium,platinum, palladium, rhodium and combinations thereof. In specificembodiments, iron is ion exchanged into the zeolite. The metal can beexchanged after manufacture of the zeolite. According to one or moreembodiments, at least a portion of the metal can be included in thetailored colloid such that the tailored colloid contains the structuredirecting agent, a silica source, and alumina source and a metal ion(e.g., copper) source.

For additional promotion of SCR of oxides of nitrogen, a suitablealkaline earth or alkali metal is exchanged into the copper promotedmolecular sieve material. Suitable alkaline earth or alkali metalsinclude, but are not limited to, barium, magnesium, beryllium, calcium,strontium, radium, and combinations thereof. In specific embodiments,the alkaline earth or alkali metal component is selected from barium,magnesium, calcium and combinations thereof. In very specificembodiments, barium is exchanged into the copper promoted molecularsieve. The metal can be exchanged after the manufacture of the molecularsieve.Iron-exchange into to alkali metal or NH₄-Chabazite to formmetal-Chabazite:

Iron is ion exchanged into alkali metal or NH4 8 ring small poremolecular sieves. In specific embodiments, iron is ion exchanged intoalkali metal or NH₄-Chabazite to form FeChabazite. According to anembodiment of the present invention, the molecular sieve material (whichmay be zeolitic material or non-zeolitic material) of the invention isused in a catalytic process, for example, as a catalyst and/or catalystsupport, and more specifically as a catalyst. In general, the molecularsieve material of the invention can be used as a catalyst and/orcatalyst support in any conceivable catalytic process, wherein processesinvolving the conversion of at least one organic compound, morespecifically of organic compounds comprising at least one carbon-carbonand/or carbon-oxygen and/or carbon-nitrogen bond, more specifically oforganic compounds comprising at least one carbon-carbon and/orcarbon-oxygen bond, and even more specifically of organic compoundscomprising at least one carbon-carbon bond. In particularly specificembodiments of the present invention, the molecular sieve material isused as a catalyst and/or catalyst support in any one or more ofmethanol-to-olefin (MTO) reactions, ethylene-to-propylene (ETP)reactions, as well as of the co-reaction of methanol and ethylene (CME).The processes involve contacting the compounds with the catalystsaccording to embodiments of the invention.

According to a further embodiment of the present invention, themolecular sieve material of the invention used in a catalytic processinvolving the conversion of at least one compound comprising at leastone nitrogen-oxygen bond. According to one or more embodiments of thepresent invention the molecular sieve material is used as a catalystand/or catalyst support in a selective catalytic reduction (SCR) processfor the selective reduction of nitrogen oxides NO_(x); for the oxidationof NH₃, in particular for the oxidation of NH₃ slip in diesel systems;for the decomposition of N₂O. The term nitrogen oxides, NO_(x), as usedin the context of the present invention designates the oxides ofnitrogen, especially dinitrogen oxide (N₂O), nitrogen monoxide (NO),dinitrogen trioxide (N₂O₃), nitrogen dioxide (NO₂), dinitrogen tetroxide(N₂O₄), dinitrogen pentoxide (N₂O₅), nitrogen peroxide (NO₃). Accordingto particularly specific embodiments of the present invention, themolecular sieve material used in a catalytic process involving theconversion of at least one compound comprising at least onenitrogen-oxygen bond comprises Fe. The process can be accomplished bycontacting the compound with a catalyst according to an embodiment ofthe invention.

Therefore, the present invention also relates to a method forselectively reducing nitrogen oxides NO_(x) by contacting a streamcontaining NO_(x) with a catalyst containing the molecular sievematerial according to the present invention under suitable reducingconditions; to a method of oxidizing NH₃, in particular of oxidizing NH;slip in diesel systems, by contacting a stream containing NH₃ with acatalyst containing the molecular sieve material having an LEV-typeframework structure according to the present invention under suitableoxidizing conditions; to a method of decomposing of N₂O by contacting astream containing N₂O with a catalyst containing the molecular sievematerial under suitable decomposition conditions; to a method ofcontrolling emissions in Advanced Emission Systems such as HomogeneousCharge Compression Ignition (HCCI) engines by contacting an emissionstream with a catalyst containing the molecular sieve material undersuitable conditions; to a fluid catalytic cracking FCC process whereinthe molecular sieve material is employed as additive; to a method ofconverting an organic compound by contacting said compound with acatalyst containing the molecular sieve material under suitableconversion conditions; to a “stationary source” process wherein acatalyst is employed containing the molecular sieve material.

Accordingly, embodiments of the present invention also relates to amethod for selectively reducing nitrogen oxides NO_(x), wherein agaseous stream containing nitrogen oxides NO_(x), specifically alsocontaining ammonia and/urea, is contacted with the molecular sievematerial according to the present invention or the molecular sievematerial obtainable or obtained according to the present invention, forexample, in the form of a molded catalyst, specifically as a moldedcatalyst wherein the molecular sieve material is deposited on a suitablerefractory carrier, still more specifically on a “honeycomb” carrier.

The nitrogen oxides which are reduced using a catalyst containing themolecular sieve material obtainable or obtained according to embodimentsof the present invention may be obtained by any process, e.g. as a wastegas stream. Among others, waste gas streams as obtained in processes forproducing adipic acid, nitric acid, hydroxylamine derivatives,caprolactame, glyoxal, methyl-glyoxal, glyoxylic acid or in processesfor burning nitrogenous materials may be mentioned.

In specific embodiments, the molecular sieve material or the molecularsieve material obtainable or obtained according to embodiments of thepresent invention is used as a molded catalyst, still more specificallyas a molded catalyst wherein the molecular sieve material is depositedon a suitable refractory carrier, still more specifically on a“honeycomb” carrier, for the selective reduction of nitrogen oxidesNO_(x), i.e. for selective catalytic reduction of nitrogen oxides. Inparticular, the selective reduction of nitrogen oxides wherein themolecular sieve material according to an embodiment of the presentinvention is employed as catalytically active material is carried out inthe presence ammonia or urea. While ammonia is the reducing agent ofchoice for stationary power plants, urea is the reducing agent of choicefor mobile SCR systems. Typically, the SCR system is integrated in theengine and vehicle design and, also typically, contains the followingmain components; SCR catalyst containing the molecular sieve materialaccording to an embodiment of the present invention; a urea storagetank; a urea pump; a urea dosing system; a urea injector/nozzle; and arespective control unit.

More specific embodiments pertain to the use of a catalyst containingthe molecular sieve material according to the present invention or themolecular sieve material obtainable or obtained according to theinventive process for removal of nitrogen oxides NO_(x) from exhaustgases of internal combustion engines, in particular diesel engines,which operate at combustion conditions with air in excess of thatrequired for stoichiometric combustion, i.e. in a lean operation mode.

Therefore, embodiments the present invention also relates to a methodfor removing nitrogen oxides NO_(x) from exhaust gases of internalcombustion engines, in particular diesel engines, which operate atcombustion conditions with air in excess of that required forstoichiometric combustion, i.e., at lean conditions, wherein a catalystcontaining the molecular sieve material according to the presentinvention or the molecular sieve material obtainable or obtainedaccording to the present invention is employed as catalytically activematerial.

Embodiments of the present invention therefore relates to the use of the8-ring small pore molecular sieve promoted with iron of the invention,in particular in the field of catalysis and/or in the treatment ofexhaust gas, wherein said exhaust gas treatment comprises industrial andautomotive exhaust gas treatment. In these and other applications, the8-ring small pore molecular sieve promoted with iron of the presentinvention can by way of example be used as a molecular sieve, catalyst,and/or catalyst support.

The invention is now described with reference to the following examples.Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways.

EXAMPLES Preparation of Catalyst Samples Example 1 Preparation of Fe-CHASamples

Iron is incorporated into the sodium CHA through Fe-ion exchange atabout 80° C. for about 2 hours at pH about 4. The mixture is then washedwith deionized water, filtered, and vacuum/air dried. Samples wereprepared targeting 1 (Example 1A), 2 (Example 1B), 3 (Example 1C), 5(Example 1D) and 10 (Example 1E) wt. % Fe loading. Washcoats wereprepared by mixing water and water and Fe zeolite to generate a target45% by weight solids slurry. The slurry is homogenous. The mixture ismixed well. The particle size is checked to ensure that D₉₀ is less than12 microns. Based on the total solids content binder is added. Themixture is mixed well. The physical properties were checked (solidcontent, pH, particle size/PSD, viscosity). If the particle size D₉₀ wasgreater than 10 microns, the slurry was milled to about 8-10 microns.

The slurry was coated onto 1″D×3″L cellular ceramic cores, having a celldensity of 400 cpsi (cells per square inch) and a wall thickness of 6.5mil. The coated cores were dried at 110° C. for 3 hours and calcined at400° C. for 1 hour. The coating process was repeated once to obtain atarget washcoat loading of 2.4 g/in³. If the slurry is not coatable, itis diluted to make it coated (minimum dilution).

Example 2 In Situ Fe-Exchanged CHA

Alternatively, an in-situ method can be used to prepare theiron-promoted 8-ring small pore molecular sieve. For example, anappropriate concentration of Fe salt solution is added drop-wise to amixing slurry of Hydrogen or ammonium form SSZ-13. The mixture is rolledovernight and milled to appropriate particle size can washcoated onhoneycomb substrate as described in Example 1. Example 2 contained 1%iron by weight.

Example 3 Testing

Nitrogen oxides selective catalytic reduction (SCR) efficiency andselectivity of a fresh catalyst core was measured by adding a feed gasmixture of 500 ppm of NO, 500 ppm of NH, 10% 0, 5% H₂O, balanced with N₂to a steady state reactor containing a 1″D×3″L catalyst core. Thereaction was carried at a space velocity of 80,000 hr⁻¹ across a 150° C.to 460° C. temperature range.

Hydrothermal stability of the catalyst was measured by hydrothermalaging of the catalyst core in the presence of 10% H₂O at 750° C. for 5hours, followed by measurement of the nitrogen oxides SCR efficiency andselectivity by the same process as outlined above for the SCR evaluationon a fresh catalyst core.

FIG. 1 shows NO_(x) conversion for Example 1A versus Example 2, eachwith 1 wt. % iron.

Table 1 shows the results.

TABLE 1 TEMPERATURE NO_(x) CONVERSION EXAMPLE # (° C.) (%) 1 (Cu-CHA)200 10 250 30 300 60 450 82 500 82 600 79

The results show stable high temperature performance of Fe-SSZ13 isobserved >400° C. Conventional liquid ion exchanged Fe-SSZ13 showshigher performance at low temperatures.

Example 4 Testing of Varying Loadings of Fe

FIG. 2 compares the NO_(x) conversion for Examples 1A, 1B and 1C,respectively containing 1, 2 and 3 wt.° % iron. The results illustratethat high temperature performance increases with Fe loading.

Example 5 Further Testing of Varying Loadings

Examples 1B (2 wt. %), 1C (3 wt. %), 1D (5 wt. %) and 1E (10 wt. %) Feloading were tested for NO_(x) conversion and N₂O concentration (or N₂Omake) exiting the catalyst. N₂O is a greenhouse gas, and it is desirablethat N2O exiting the catalyst is as low as possible. FIG. 4 shows NOxconversion results. Samples containing 5% and 10% showed significantlybetter NOx conversion at the lower temperature region of 200° C. to 350°C., as well as the high temperature region of 350° C. 5o 600° C. At 550°C., the NOx conversion of the 10% Fe-loaded sample was several percenthigher.

FIG. 4 shows the dramatic improvement in the reduction in N2O forExamples 1D and 1E, including 5% and higher Fe.

Reference throughout this specification to “one embodiment,” “certainembodiments,” “one or more embodiments” or “an embodiment” means that aparticular feature, structure, material, or characteristic described inconnection with the embodiment is included in at least one embodiment ofthe invention. Thus, the appearances of the phrases such as “in one ormore embodiments,” “in certain embodiments,” “in one embodiment” or “inan embodiment” in various places throughout this specification are notnecessarily referring to the same embodiment of the invention.Furthermore, the particular features, structures, materials, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It will be apparent to those skilled in the art thatvarious modifications and variations can be made to the method andapparatus of the present invention without departing from the spirit andscope of the invention. Thus, it is intended that the present inventioninclude modifications and variations that are within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A selective catalytic reduction catalystcomprising an 8-ring small pore molecular sieve promoted with greaterthan 5 wt. % iron so that the catalyst is effective to catalyze theselective catalytic reduction of nitrogen oxides in the presence of areductant at temperatures between 200° and 600° C.
 2. The catalyst ofclaim 1, wherein the iron-promoted 8-ring small pore molecular sieve isselected from the group consisting of iron-promoted zeolite having astructure type selected from AEI, AFT, AFX, CHA, EAB, ERI, KFI, LEV,SAS, SAT, and SAV.
 3. The catalyst of claim 2, wherein the iron-promoted8-ring small pore molecular sieve has the CHA crystal structure.
 4. Thecatalyst of claim 3, wherein the iron-promoted 8-ring small poremolecular sieve having the CHA crystal structure is selected from analuminosilicate zeolite, a borosilicate, a gallosilicate, a SAPO, anALPO, a MeAPSO, and a MeAPO.
 5. The catalyst of claim 4, wherein theiron-promoted 8-ring small pore molecular sieve is selected from thegroup consisting of SSZ-13, SSZ-62, natural chabazite, zeolite K-G,Linde D, Linde R, LZ-218, LZ-235, LZ-236, ZK-14, SAPO-34, SAPO-44,SAPO-47, and ZYT-6.
 6. The catalyst of claim 3, wherein the 8-ring smallpore molecular sieve having the CHA structure is an aluminosilicatezeolite having the CHA crystal structure.
 7. The catalyst of claim 6,wherein the aluminosilicate zeolite having the CHA crystal structure isselected from iron-promoted SSZ-13 and iron-promoted SSZ-62.
 8. Thecatalyst of claim 7, wherein the zeolite has a silica to alumina ratioin the range of 5 and
 100. 9. The catalyst of claim 8, wherein thezeolite has a silica to alumina ratio in the range of 10 and
 50. 10. Thecatalyst of claim 1, comprising iron in the range of 5.1 wt. % to 10 wt.%, calculated as Fe₂O₃.
 11. The catalyst of claim 7, comprising iron inthe range of t 5.1 wt. % to 10 wt. %, calculated as Fe₂O₃.
 12. Thecatalyst of claim 8, comprising iron in the range of 5.1 wt. % to 10 wt.%, calculated as Fe₂O₃.
 13. A catalytic article comprising the catalystof claim 1 in a washcoat deposited on a honeycomb substrate.
 14. Thecatalytic article of claim 13, wherein the honeycomb substrate comprisesa wall flow filter substrate.
 15. The catalytic article of claim 13,wherein the honeycomb substrate comprises a flow through substrate. 16.An exhaust gas treatment system comprising the catalytic article ofclaim 13 disposed downstream from a diesel engine and an injector thatadds a reductant to an exhaust gas stream from the engine.
 17. A methodfor selectively reducing nitrogen oxides, the method comprisingcontacting a gaseous stream containing nitrogen oxides with a selectivecatalytic reduction catalyst comprising an 8-ring small pore molecularsieve promoted with greater than 5 wt. % iron to catalyze the selectivecatalytic reduction of the nitrogen oxides in the presence of areductant at temperatures between 200° and 600° C.
 18. The method ofclaim 17, wherein the 8-ring small pore molecular sieve promoted withiron is selected from the group consisting of AEI, AFT, AFX, CHA, EAB,ERI, KFI, LEV, SAS, SAT, and SAV.
 19. The method of claim 18, whereinthe 8-ring small pore molecular sieve promoted with iron has the CHAcrystal structure.
 20. The method of claim 19, wherein the 8-ring smallpore molecular sieve promoted with iron and having the CHA crystalstructure is selected from the group consisting of aluminosilicatezeolite, SAPO, ALPO and MeAPO.